Selective call receiving devices are known that include a peak and valley detector to detect the peak and valley of a received signal. Values representing a detected peak and valley are used to derive thresholds to which an incoming signal is compared in order to decode the signal into symbols such as 0, 1 for a two-level signal or into symbols such as 00, 01, 10, 11 for a four-level signal. Examples of known peak detectors are described in U.S. Pat. Nos. 4,631,737 and 5,025,251.
Although known peak detector schemes can adequately detect and track a peak and valley of a two-level signal, four-level signals require even more accurate peak and valley detection and tracking. Problems have arisen when peak and valley detections schemes designed for two-level signals are used for four-level signals. For example, if the incoming signal represents a long string of symbols corresponding to the mid-levels of a four-level signal, peak and valley detectors have been known to track the peak and valley to the mid-level signals as opposed to the outer level signals where the true peak and valley occur. Inadequate tracking of a four-level signal can also occur if the attack and decay of the peak and valley are not accomplished independently.